HD5770, 5830, 5850, 5870, 6870, 6850, 7770, umm.... craploads of cards out there for under 400, you could get three 5830s for that much, or two 5850s and a 5770, a little bit more would get you two 5850s and a 5830

CPU and ram don't need to be upgraded, unless you're gaming, then it won't hurt, don't use crossfire, use each card separately, power supply probably needs to be upgraded, most prebuilt systems come with shitty power supplies that can only provide the bare minimum of what the system will need

I wouldnt put more than 1 graphics card on that power supply. It -should- be capable delivering power to just about any single gpu card though if the video card has proper core/memory clocks and core voltage. I got a junky 300W power supply that powers a radeon 5850 @ 1015MHz and stock volts just fine.

Anyways, you can probably pick up a radeon 5830 for about $100. If you undervolt it to 0.95v, you may be looking at about 235 mhash/sec. If you leave it at stock volts, you might be looking at 310 mhash/sec. You might be able to pick up a radeon 5870 which is good for about 335 mhash/sec when undervolted to 0.95v, or 450 mhash/sec with stock volts. Newer generation cards will probably cost you much more. A radoen 7970 will probably set you back about $400 and you're probably looking at 500 mhash when undervolted, or 700 mhash at stock volts.

See those short black slots, with a 1x to 16x riser cable, you can put graphics cards on them, but you'd have to find a way to mount them on a frame or something, wood is cheap and easy to work with

And yea, you'll only be able to run one gfx card on that power supply, look for a 800W psu at least, another thing to keep in mind is PCIE power connectors, there are Molex to PCIE converters which take two Molex connectors and make them one PCIE connector